This invention relates to a method, used in a spent nuclear fuel retreatment process, of rendering mineral an iodine compound containing recovered radioactive iodine and storing it, the radioactive iodine being recovered as an iodine compound without causing release of long half-life .sup.129 I radioactive iodine into the environment, and disposing of scrub liquids from which radioactive iodine has been collected through freeze vacuum drying.
In a spent nuclear fuel retreatment process, elimination of radioactive iodine generated in the process of dissolving and shearing spent fuel is particularly vital due to a high iodine toxicity, a high iodine content of fission products, and a high rate of nuclear fission realized in the mass index of the two primary types of radioactive iodines, 1% .sup.129 I at a long (1.7.times.10.sup.7 years) half-life and 2.09% .sup.131 I, a brief (eight day) half-life. When the iodine in the liquids supplied to solvent extraction remains to react with the solvent, it produces a chemical compound resistant to removal with a resultant contamination throughout the entire treatment process and makes important the removal of as much iodine as is practicable before it comes into contact with the solvent.
In the off-gas phase, aqueous phase or organic phase, iodine appears in I.sub.2, HI or HIO or organic forms and appears as HIO.sub.3 in dense nitrate solutions.
Conventional methods of removing iodine are removal by immersion in alkaline solutions such as Na.sub.2 CO.sub.3 or NaOH or removal also through use of silver zeolite loaded with silver in a zeolite catalyst.
In methods employing alkaline solutions, however, removal of organic iodine is impossible and there is furthermore no way to recover iodine and treat it while maintaining it in solution. Radioactive iodine incapable of maintenance is released into the atmosphere and the accumulation of .sup.129 I particularly with its long half life becomes a problem. In recovering radioactive iodine it is further necessary to utilize such valuable absorbents as silver zeolite. For this purpose a process making possible the suspension of radioactive iodine in a liquid is desirable. As it is vital to store long half-life .sup.129 I over long periods, a process making possible safe storage of radioactive materials such as recovered .sup.129 I is desired. In addition, liquid waste containing radioactive iodines generated in the retreatment process does not cause vaporization of radioactive iodines, so it is necessary to control the alkalinity. However, if this liquid waste becomes mixed with acidic liquid wastes, there is the danger that radioactive iodine may be irretrievably released. For this reason, there is need of a process in which liquid containing radioactive iodines may be treated individually without becoming mixed in with liquid wastes from another process.